Block <a> tags are getting stripped from the Editor

[georgestephanis]

Steps to duplicate:

Make a new post.
Have a single line of text saying 'Foo foo foo'.
Change it to a h1 block.
Select the line.
Add a hyperlink to example.com
Switch to the Text tab.
Switch back to the Visual Editor.

THE LINK IS GONE ZOMG WUT?!?!

Basically since we've incorporated the HTML5 Schema for TinyMCE, <a> tags have been upgraded to a block-level element. So TinyMCE will wrap the <h1> tag with the <a> tag -- as opposed to the opposite in 3.4.

Then we go in and strip it out (because reasons).

Let's stop stripping out valid tags that we're adding via the editor in 3.5.1